Efficient thin film devices, including organic photovoltaic and organic electroluminescent devices have been the subject of much advancement recently. In particular, organic light emitting materials have attracted increasing interest in the past two decades.1 OLEDs may be either small molecule OLED or polymer OLED. Soluble light emitting polymers are appealing for manufacture of large area, low cost light emitting devices.2 In recent years, screen and inkjet printing have been successfully applied to this area, promising lower cost for area lighting, patterning and display applications.3 While the emissive electroluminescent layer is polymeric, varying amounts of OLEDs can be deposited in arrays on a screen using simple “printing” methods to create a graphical color display, for use as television screens, computer displays, advertising and information board applications, and the like. OLED may also be used in lighting devices. Prior to standardization, OLED technology was also referred to as organic electro-luminescence.
In order to achieve low cost and high efficiency, it is crucial to have an air stable cathode with efficient electron injection properties. Currently, OLEDs are processed using a cathode that is very sensitive to oxygen and water. The electron injection for the polymer device is achieved by using a low work function metal such as calcium or magnesium, which are very reactive to oxygen and moisture and become poor conductors due to its oxidation. Consequently, the manufacturing of these devices requires processing under vacuum while applying the cathode and subsequent application of an air impermeable encapsulation to ensure life of the product. There are also silver paint based polymer light emitted diode devices, however, those devices are used in electrochemical cells rather than OLEDs. Additionally, the use of metals such as calcium, magnesium, or silver blocks useable light from the cathode since they are not transparent and trap the light emissions within the polymer device.
Single wall carbon nanotubes (SVVNT) have been demonstrated to be viable as electron injection material for application in OLEDs.4 However, the applications are limited by the available materials to render the SWNT soluble as well as its compatibility to the light emitting materials. SWNTs commonly aggregate in solution which may render them unusable since it is not possible to form a uniform layer of the SWNTs on the cathode and/or many SWNT aggregates must be used in order to be a good conductor. Thus, surfactants are used to grab the nanotubes and disperse them. Unfortunately, poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-vinylene) (PmPV) is so far the only conducting polymer which has been found to be able to wrap around the SWNT to stabilize the tubes in solution phase.5 Low solubility of SWNT in other conductive polymer systems is one of the major obstacles for wide range applications.
Other viable materials found to solubilize/suspend SWNT in water solution is polyvinyl pyrrolidone (PVP) and polystyrene sulfonate (PSS). Small molecule amphiphiles (surfactants) have also been used to solublize/suspend SWNT in aqueous solutions.
The established methods of dissolving carbon nanotubes are either using surfactants or polymer wrapping agents.6 Those molecules are able to associate on the nanotube surface. The carbon nanotube concentrations are typically 0.1-0.01% (by weight) in these surfactant stabilized systems. In situ polymerization of the surfactant in the solution system can provide a permanent coating on the nanotube. Therefore, development of other conductive polymer surfactants to disperse the SWNTs and non-metal based OLEDs that are processed with air stable cathode composite materials with good electron injection properties is a key step for lowering the overall cost of OLEDs.